Now, devices in which thin films are intentionally formed on a substrate for various purposes are manufactured on an industrial basis. Also, the thin films on the substrate, which are accidentally formed, may change the characteristics of the devices or material. For example, as semiconductor devices or magnetic head devices are miniaturized, the processing sizes (film thickness) that influence the device performance and a required precision in the film quality are extremely demanded more and more. When the semiconductor device is miniaturized as described above, and the design rule is set to 90 nm or lower, the gate oxide film of a semiconductor transistor is extremely thinned to about 1 nm. The film thickness greatly depends on physical characteristics such as a leak current or a dielectric constant within the transistor. Also, in the device structure of the magnetic head device, a magnetoresistance effect due to the spin dependent scattering of electrons that pass through a nonmagnetic layer or a barrier layer is utilized in any head structure of the nm structure. The head sensitivity largely depends on the film thickness. For that reason, a technique by which the film thickness of sub nm is evaluated with a high precision is one of important issues in developing the next generation of heads or mass-producing the heads with a stable sensitivity.
Up to now, an x-ray reflectometry instrument has been employed in the device for measuring the length of the thin films, as disclosed in Japanese Patent Laid-Open No. H7 (1995)-311165. In the case of the x-ray reflectometry instrument, a specimen resulting from laminating thin films in multi-layers is irradiated with x-rays to measure the x-ray reflectivity, the obtained patterns are analyzed, and an attempt is made to absolutely evaluate information on the thickness and reflectivity of the thin films and the density. Furthermore, this method using x-rays is advantageous in that the method can be conducted in the atmosphere, the method can be applied to an optically opaque system such as metal, and a nondestructive evaluation can be conducted.
Also, as a thin film evaluating method within a micro-area, there is conducted a direct observation using the transmission electron microscope. The thicknesses of films are measured from the photographic image by means of the transmission electron microscope as follows: First, a position of the specimen in an optically axial direction and a current value of an objective lens are fixed, the specimen having known dimensions is observed at a given magnification under predetermined conditions. Then, the excitation current of plural imaging lenses is adjusted in such a manner that a photographic image on an image display substantially coincides with the display magnification. The excitation conditions of the imaging lenses at that time are stored in association with the display magnification.
The same operation is conducted on other setting display magnifications, and the excitation conditions of the imaging lens corresponding to the respective display magnifications are recorded and saved, respectively. Setting the magnification is conducted by a moiré pattern or a crystal lattice image which is capable of determining the intervals through the electron diffraction method.
In the case of measuring an arbitrary film thickness within the photographic image, after the excitation conditions of the imaging lenses which are stored with respect to a certain magnification are reproduced, the photographic image is displayed on the image display, the sizes within the display image are measured, and an actual thickness is displayed according to the measurement magnification.
As described above, in the case of using x-rays in measurement of the thickness of the thin. film, the x-rays with which the specimen is irradiated cannot be focused more than about several micrometers. Therefore, measurement can be conducted when the film structure has the same lamination over the wide area, but the thickness of the thin film cannot be measured in the case where the film structure is identical in only the nanometers area.
The reason that an object can be observed with a transmission electron microscope image is because there is contrast in the electron intensity within the photographic image. Because the specimen which is observed by the transmission electron microscope is very thin in the transmission direction of electron beams, most of incident electrons are allowed to penetrate the interior of the specimen. In this case, the transmission electron microscope image uniformly brightly appears. However, the reason that an object can be observed with the transmission electron microscope image is because electrons are scattered within the specimen transmits the electrons. The intensity at which the electrons are scattered is attributed to the atomic scattering factor of the respective atoms within the specimen. For that reason, in the case where the atomic numbers of the elements that constitute the adjacent objects or films are close to each other in the visual field of observation, the atomic scattering factors are also values very close to each other, as a result of which there is little contrast of the transmission electron microscope image.
In recent years, there are analyzing manners such as an electron energy loss spectroscopy (EELS) in which the specimen is irradiated with electron beams, and loss transmission electrons are sorted into energies due to the mutual interaction between the incident electrons and the atoms within the specimen, and an energy dispersive x-ray spectroscopy (EDS or EDX) in which the characteristic x-ray that is generated from the specimen is analyzed by a semiconductor detector, in the transmission electron microscope or scanning transmission electron microscope each having an analyzer. Through those manners, the elemental maps that select the specific elements within the micro portion of the specimen are acquired, and an attempt is made to measure the arbitrary distances from the image. Since those analyzing manners are very high in spatial resolution to the degree of from several nanometers to several tens nanometers, and also selects only the elements to be observed, the contrast within the obtained image is sharp. However, there arises a problem on the imprecision of the length measurement function depending on the measurement conditions or the selected elements.
Under the above circumstances, the present invention has been made to solve the problems with the above-mentioned conventional film thickness measuring method and the evaluating apparatus. Therefore an object of the present invention is to measure the thickness of an ultra thin film with a high precision. Another object of the present invention is to a method and apparatus which are capable of measuring the film thickness in the thin film structure having no micro area or crystal structure.